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<span class="pre noprint docinfo">[<a href="https://www.rfc-editor.org" title="RFC Editor">RFC Home</a>] [<a href="/rfc/rfc7228.txt">TEXT</a>|<a href="/rfc/pdfrfc/rfc7228.txt.pdf">PDF</a>|<a href="/rfc/rfc7228.html">HTML</a>] [<a href='https://datatracker.ietf.org/doc/rfc7228' title='IETF Datatracker information for this document'>Tracker</a>] [<a href="https://datatracker.ietf.org/ipr/search/?rfc=7228&amp;submit=rfc" title="IPR disclosures related to this document">IPR</a>] [<a href='https://www.rfc-editor.org/info/rfc7228' title='Info page'>Info page</a>]                  </span><br/><span class="pre noprint docinfo">                                                                        </span><br /><span class="pre noprint docinfo">                                                           INFORMATIONAL</span><br /><span class="pre noprint docinfo">                                                                        </span><pre>Internet Engineering Task Force (IETF)                        C. Bormann
Request for Comments: 7228                       Universitaet Bremen TZI
Category: Informational                                         M. Ersue
ISSN: 2070-1721                             Nokia Solutions and Networks
                                                              A. Keranen
                                                                Ericsson
                                                                May 2014


               <span class="h1">Terminology for Constrained-Node Networks</span>

Abstract

   The Internet Protocol Suite is increasingly used on small devices
   with severe constraints on power, memory, and processing resources,
   creating constrained-node networks.  This document provides a number
   of basic terms that have been useful in the standardization work for
   constrained-node networks.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see <a href="./rfc5741#section-2">Section&nbsp;2 of RFC 5741</a>.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   <a href="http://www.rfc-editor.org/info/rfc7228">http://www.rfc-editor.org/info/rfc7228</a>.

















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<span class="grey"><a href="./rfc7228">RFC 7228</a>                     CNN Terminology                    May 2014</span>


Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
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   (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   <a href="#section-1">1</a>. Introduction ....................................................<a href="#page-3">3</a>
   <a href="#section-2">2</a>. Core Terminology ................................................<a href="#page-4">4</a>
      <a href="#section-2.1">2.1</a>. Constrained Nodes ..........................................<a href="#page-4">4</a>
      <a href="#section-2.2">2.2</a>. Constrained Networks .......................................<a href="#page-5">5</a>
           <a href="#section-2.2.1">2.2.1</a>. Challenged Networks .................................<a href="#page-6">6</a>
      <a href="#section-2.3">2.3</a>. Constrained-Node Networks ..................................<a href="#page-7">7</a>
           <a href="#section-2.3.1">2.3.1</a>. LLN .................................................<a href="#page-7">7</a>
           <a href="#section-2.3.2">2.3.2</a>. LoWPAN, 6LoWPAN .....................................<a href="#page-8">8</a>
   <a href="#section-3">3</a>. Classes of Constrained Devices ..................................<a href="#page-8">8</a>
   <a href="#section-4">4</a>. Power Terminology ..............................................<a href="#page-10">10</a>
      <a href="#section-4.1">4.1</a>. Scaling Properties ........................................<a href="#page-10">10</a>
      <a href="#section-4.2">4.2</a>. Classes of Energy Limitation ..............................<a href="#page-11">11</a>
      <a href="#section-4.3">4.3</a>. Strategies for Using Power for Communication ..............<a href="#page-12">12</a>
   <a href="#section-5">5</a>. Security Considerations ........................................<a href="#page-14">14</a>
   <a href="#section-6">6</a>. Acknowledgements ...............................................<a href="#page-14">14</a>
   <a href="#section-7">7</a>. Informative References .........................................<a href="#page-14">14</a>


















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<span class="grey"><a href="./rfc7228">RFC 7228</a>                     CNN Terminology                    May 2014</span>


<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   Small devices with limited CPU, memory, and power resources, so-
   called "constrained devices" (often used as sensors/actuators, smart
   objects, or smart devices) can form a network, becoming "constrained
   nodes" in that network.  Such a network may itself exhibit
   constraints, e.g., with unreliable or lossy channels, limited and
   unpredictable bandwidth, and a highly dynamic topology.

   Constrained devices might be in charge of gathering information in
   diverse settings, including natural ecosystems, buildings, and
   factories, and sending the information to one or more server
   stations.  They might also act on information, by performing some
   physical action, including displaying it.  Constrained devices may
   work under severe resource constraints such as limited battery and
   computing power, little memory, and insufficient wireless bandwidth
   and ability to communicate; these constraints often exacerbate each
   other.  Other entities on the network, e.g., a base station or
   controlling server, might have more computational and communication
   resources and could support the interaction between the constrained
   devices and applications in more traditional networks.

   Today, diverse sizes of constrained devices with different resources
   and capabilities are becoming connected.  Mobile personal gadgets,
   building-automation devices, cellular phones, machine-to-machine
   (M2M) devices, and other devices benefit from interacting with other
   "things" nearby or somewhere in the Internet.  With this, the
   Internet of Things (IoT) becomes a reality, built up out of uniquely
   identifiable and addressable objects (things).  Over the next decade,
   this could grow to large numbers [<a href="#ref-FIFTY-BILLION">FIFTY-BILLION</a>] of Internet-
   connected constrained devices, greatly increasing the Internet's size
   and scope.

   The present document provides a number of basic terms that have been
   useful in the standardization work for constrained environments.  The
   intention is not to exhaustively cover the field but to make sure a
   few core terms are used consistently between different groups
   cooperating in this space.

   In this document, the term "byte" is used in its now customary sense
   as a synonym for "octet".  Where sizes of semiconductor memory are
   given, the prefix "kibi" (1024) is combined with "byte" to
   "kibibyte", abbreviated "KiB", for 1024 bytes [<a href="#ref-ISQ-13" title="&quot;International Standard -- Quantities and units -- Part 13: Information science and technology&quot;">ISQ-13</a>].








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<span class="grey"><a href="./rfc7228">RFC 7228</a>                     CNN Terminology                    May 2014</span>


   In computing, the term "power" is often used for the concept of
   "computing power" or "processing power", as in CPU performance.  In
   this document, the term stands for electrical power unless explicitly
   stated otherwise.  "Mains-powered" is used as a shorthand for being
   permanently connected to a stable electrical power grid.

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Core Terminology</span>

   There are two important aspects to _scaling_ within the Internet of
   Things:

   o  scaling up Internet technologies to a large number [<a href="#ref-FIFTY-BILLION">FIFTY-BILLION</a>]
      of inexpensive nodes, while

   o  scaling down the characteristics of each of these nodes and of the
      networks being built out of them, to make this scaling up
      economically and physically viable.

   The need for scaling down the characteristics of nodes leads to
   "constrained nodes".

<span class="h3"><a class="selflink" id="section-2.1" href="#section-2.1">2.1</a>.  Constrained Nodes</span>

   The term "constrained node" is best defined by contrasting the
   characteristics of a constrained node with certain widely held
   expectations on more familiar Internet nodes:

   Constrained Node:  A node where some of the characteristics that are
      otherwise pretty much taken for granted for Internet nodes at the
      time of writing are not attainable, often due to cost constraints
      and/or physical constraints on characteristics such as size,
      weight, and available power and energy.  The tight limits on
      power, memory, and processing resources lead to hard upper bounds
      on state, code space, and processing cycles, making optimization
      of energy and network bandwidth usage a dominating consideration
      in all design requirements.  Also, some layer-2 services such as
      full connectivity and broadcast/multicast may be lacking.

   While this is not a rigorous definition, it is grounded in the state
   of the art and clearly sets apart constrained nodes from server
   systems, desktop or laptop computers, powerful mobile devices such as
   smartphones, etc.  There may be many design considerations that lead
   to these constraints, including cost, size, weight, and other scaling
   factors.







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<span class="grey"><a href="./rfc7228">RFC 7228</a>                     CNN Terminology                    May 2014</span>


   (An alternative term, when the properties as a network node are not
   in focus, is "constrained device".)

   There are multiple facets to the constraints on nodes, often applying
   in combination, for example:

   o  constraints on the maximum code complexity (ROM/Flash),

   o  constraints on the size of state and buffers (RAM),

   o  constraints on the amount of computation feasible in a period of
      time ("processing power"),

   o  constraints on the available power, and

   o  constraints on user interface and accessibility in deployment
      (ability to set keys, update software, etc.).

   <a href="#section-3">Section 3</a> defines a small number of interesting classes ("class-N"
   for N = 0, 1, 2) of constrained nodes focusing on relevant
   combinations of the first two constraints.  With respect to available
   power, [<a href="./rfc6606" title="&quot;Problem Statement and Requirements for IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) Routing&quot;">RFC6606</a>] distinguishes "power-affluent" nodes (mains-powered
   or regularly recharged) from "power-constrained nodes" that draw
   their power from primary batteries or by using energy harvesting;
   more detailed power terminology is given in <a href="#section-4">Section 4</a>.

   The use of constrained nodes in networks often also leads to
   constraints on the networks themselves.  However, there may also be
   constraints on networks that are largely independent from those of
   the nodes.  We therefore distinguish "constrained networks" from
   "constrained-node networks".

<span class="h3"><a class="selflink" id="section-2.2" href="#section-2.2">2.2</a>.  Constrained Networks</span>

   We define "constrained network" in a similar way:

   Constrained Network:  A network where some of the characteristics
      pretty much taken for granted with link layers in common use in
      the Internet at the time of writing are not attainable.

   Constraints may include:

   o  low achievable bitrate/throughput (including limits on duty
      cycle),

   o  high packet loss and high variability of packet loss (delivery
      rate),




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<span class="grey"><a href="./rfc7228">RFC 7228</a>                     CNN Terminology                    May 2014</span>


   o  highly asymmetric link characteristics,

   o  severe penalties for using larger packets (e.g., high packet loss
      due to link-layer fragmentation),

   o  limits on reachability over time (a substantial number of devices
      may power off at any point in time but periodically "wake up" and
      can communicate for brief periods of time), and

   o  lack of (or severe constraints on) advanced services such as IP
      multicast.

   More generally, we speak of constrained networks whenever at least
   some of the nodes involved in the network exhibit these
   characteristics.

   Again, there may be several reasons for this:

   o  cost constraints on the network,

   o  constraints posed by the nodes (for constrained-node networks),

   o  physical constraints (e.g., power constraints, environmental
      constraints, media constraints such as underwater operation,
      limited spectrum for very high density, electromagnetic
      compatibility),

   o  regulatory constraints, such as very limited spectrum availability
      (including limits on effective radiated power and duty cycle) or
      explosion safety, and

   o  technology constraints, such as older and lower-speed technologies
      that are still operational and may need to stay in use for some
      more time.

<span class="h4"><a class="selflink" id="section-2.2.1" href="#section-2.2.1">2.2.1</a>.  Challenged Networks</span>

   A constrained network is not necessarily a "challenged network"
   [<a href="#ref-FALL" title="&quot;A Delay-Tolerant Network Architecture for Challenged Internets&quot;">FALL</a>]:

   Challenged Network:  A network that has serious trouble maintaining
      what an application would today expect of the end-to-end IP model,
      e.g., by:

      *  not being able to offer end-to-end IP connectivity at all,

      *  exhibiting serious interruptions in end-to-end IP connectivity,
         or



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<span class="grey"><a href="./rfc7228">RFC 7228</a>                     CNN Terminology                    May 2014</span>


      *  exhibiting delay well beyond the Maximum Segment Lifetime (MSL)
         defined by TCP [<a href="./rfc0793" title="&quot;Transmission Control Protocol&quot;">RFC0793</a>].

   All challenged networks are constrained networks in some sense, but
   not all constrained networks are challenged networks.  There is no
   well-defined boundary between the two, though.  Delay-Tolerant
   Networking (DTN) has been designed to cope with challenged networks
   [<a href="./rfc4838" title="&quot;Delay-Tolerant Networking Architecture&quot;">RFC4838</a>].

<span class="h3"><a class="selflink" id="section-2.3" href="#section-2.3">2.3</a>.  Constrained-Node Networks</span>

   Constrained-Node Network:  A network whose characteristics are
      influenced by being composed of a significant portion of
      constrained nodes.

   A constrained-node network always is a constrained network because of
   the network constraints stemming from the node constraints, but it
   may also have other constraints that already make it a constrained
   network.

   The rest of this subsection introduces two additional terms that are
   in active use in the area of constrained-node networks, without an
   intent to define them: LLN and (6)LoWPAN.

<span class="h4"><a class="selflink" id="section-2.3.1" href="#section-2.3.1">2.3.1</a>.  LLN</span>

   A related term that has been used to describe the focus of the IETF
   ROLL working group is "Low-Power and Lossy Network (LLN)".  The ROLL
   (Routing Over Low-Power and Lossy) terminology document [<a href="./rfc7102" title="&quot;Terms Used in Routing for Low-Power and Lossy Networks&quot;">RFC7102</a>]
   defines LLNs as follows:

      LLN: Low-Power and Lossy Network.  Typically composed of many
      embedded devices with limited power, memory, and processing
      resources interconnected by a variety of links, such as IEEE
      802.15.4 or low-power Wi-Fi.  There is a wide scope of application
      areas for LLNs, including industrial monitoring, building
      automation (heating, ventilation, and air conditioning (HVAC),
      lighting, access control, fire), connected home, health care,
      environmental monitoring, urban sensor networks, energy
      management, assets tracking, and refrigeration.

   Beyond that, LLNs often exhibit considerable loss at the physical
   layer, with significant variability of the delivery rate, and some
   short-term unreliability, coupled with some medium-term stability
   that makes it worthwhile to both construct directed acyclic graphs
   that are medium-term stable for routing and do measurements on the
   edges such as Expected Transmission Count (ETX) [<a href="./rfc6551" title="&quot;Routing Metrics Used for Path Calculation in Low-Power and Lossy Networks&quot;">RFC6551</a>].  Not all
   LLNs comprise low-power nodes [<a href="#ref-RPL-DEPLOYMENT">RPL-DEPLOYMENT</a>].



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<span class="grey"><a href="./rfc7228">RFC 7228</a>                     CNN Terminology                    May 2014</span>


   LLNs typically are composed of constrained nodes; this leads to the
   design of operation modes such as the "non-storing mode" defined by
   RPL (the IPv6 Routing Protocol for Low-Power and Lossy Networks
   [<a href="./rfc6550" title="&quot;RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks&quot;">RFC6550</a>]).  So, in the terminology of the present document, an LLN
   is a constrained-node network with certain network characteristics,
   which include constraints on the network as well.

<span class="h4"><a class="selflink" id="section-2.3.2" href="#section-2.3.2">2.3.2</a>.  LoWPAN, 6LoWPAN</span>

   One interesting class of a constrained network often used as a
   constrained-node network is "LoWPAN" [<a href="./rfc4919" title="&quot;IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs): Overview, Assumptions, Problem Statement, and Goals&quot;">RFC4919</a>], a term inspired from
   the name of an IEEE 802.15.4 working group (low-rate wireless
   personal area networks (LR-WPANs)).  The expansion of the LoWPAN
   acronym, "Low-Power Wireless Personal Area Network", contains a hard-
   to-justify "Personal" that is due to the history of task group naming
   in IEEE 802 more than due to an orientation of LoWPANs around a
   single person.  Actually, LoWPANs have been suggested for urban
   monitoring, control of large buildings, and industrial control
   applications, so the "Personal" can only be considered a vestige.
   Occasionally, the term is read as "Low-Power Wireless Area Networks"
   [<a href="#ref-WEI" title="&quot;6LoWPAN: the Wireless Embedded Internet&quot;">WEI</a>].  Originally focused on IEEE 802.15.4, "LoWPAN" (or when used
   for IPv6, "6LoWPAN") also refers to networks built from similarly
   constrained link-layer technologies [<a href="#ref-V6-BTLE" title="&quot;Transmission of IPv6 Packets over BLUETOOTH Low Energy&quot;">V6-BTLE</a>] [<a href="#ref-V6-DECT-ULE">V6-DECT-ULE</a>]
   [<a href="#ref-V6-G9959" title="&quot;Transmission of IPv6 packets over ITU-T G.9959 Networks&quot;">V6-G9959</a>].

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Classes of Constrained Devices</span>

   Despite the overwhelming variety of Internet-connected devices that
   can be envisioned, it may be worthwhile to have some succinct
   terminology for different classes of constrained devices.  In this
   document, the class designations in Table 1 may be used as rough
   indications of device capabilities:

     +-------------+-----------------------+-------------------------+
     | Name        | data size (e.g., RAM) | code size (e.g., Flash) |
     +-------------+-----------------------+-------------------------+
     | Class 0, C0 | &lt;&lt; 10 KiB             | &lt;&lt; 100 KiB              |
     |             |                       |                         |
     | Class 1, C1 | ~ 10 KiB              | ~ 100 KiB               |
     |             |                       |                         |
     | Class 2, C2 | ~ 50 KiB              | ~ 250 KiB               |
     +-------------+-----------------------+-------------------------+

        Table 1: Classes of Constrained Devices (KiB = 1024 bytes)

   As of the writing of this document, these characteristics correspond
   to distinguishable clusters of commercially available chips and
   design cores for constrained devices.  While it is expected that the



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   boundaries of these classes will move over time, Moore's law tends to
   be less effective in the embedded space than in personal computing
   devices: gains made available by increases in transistor count and
   density are more likely to be invested in reductions of cost and
   power requirements than into continual increases in computing power.

   Class 0 devices are very constrained sensor-like motes.  They are so
   severely constrained in memory and processing capabilities that most
   likely they will not have the resources required to communicate
   directly with the Internet in a secure manner (rare heroic, narrowly
   targeted implementation efforts notwithstanding).  Class 0 devices
   will participate in Internet communications with the help of larger
   devices acting as proxies, gateways, or servers.  Class 0 devices
   generally cannot be secured or managed comprehensively in the
   traditional sense.  They will most likely be preconfigured (and will
   be reconfigured rarely, if at all) with a very small data set.  For
   management purposes, they could answer keepalive signals and send on/
   off or basic health indications.

   Class 1 devices are quite constrained in code space and processing
   capabilities, such that they cannot easily talk to other Internet
   nodes employing a full protocol stack such as using HTTP, Transport
   Layer Security (TLS), and related security protocols and XML-based
   data representations.  However, they are capable enough to use a
   protocol stack specifically designed for constrained nodes (such as
   the Constrained Application Protocol (CoAP) over UDP [<a href="#ref-COAP" title="&quot;Constrained Application Protocol (CoAP)&quot;">COAP</a>]) and
   participate in meaningful conversations without the help of a gateway
   node.  In particular, they can provide support for the security
   functions required on a large network.  Therefore, they can be
   integrated as fully developed peers into an IP network, but they need
   to be parsimonious with state memory, code space, and often power
   expenditure for protocol and application usage.

   Class 2 devices are less constrained and fundamentally capable of
   supporting most of the same protocol stacks as used on notebooks or
   servers.  However, even these devices can benefit from lightweight
   and energy-efficient protocols and from consuming less bandwidth.
   Furthermore, using fewer resources for networking leaves more
   resources available to applications.  Thus, using the protocol stacks
   defined for more constrained devices on Class 2 devices might reduce
   development costs and increase the interoperability.

   Constrained devices with capabilities significantly beyond Class 2
   devices exist.  They are less demanding from a standards development
   point of view as they can largely use existing protocols unchanged.
   The present document therefore does not make any attempt to define
   classes beyond Class 2.  These devices can still be constrained by a
   limited energy supply.



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   With respect to examining the capabilities of constrained nodes,
   particularly for Class 1 devices, it is important to understand what
   type of applications they are able to run and which protocol
   mechanisms would be most suitable.  Because of memory and other
   limitations, each specific Class 1 device might be able to support
   only a few selected functions needed for its intended operation.  In
   other words, the set of functions that can actually be supported is
   not static per device type: devices with similar constraints might
   choose to support different functions.  Even though Class 2 devices
   have some more functionality available and may be able to provide a
   more complete set of functions, they still need to be assessed for
   the type of applications they will be running and the protocol
   functions they would need.  To be able to derive any requirements,
   the use cases and the involvement of the devices in the application
   and the operational scenario need to be analyzed.  Use cases may
   combine constrained devices of multiple classes as well as more
   traditional Internet nodes.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Power Terminology</span>

   Devices not only differ in their computing capabilities but also in
   available power and/or energy.  While it is harder to find
   recognizable clusters in this space, it is still useful to introduce
   some common terminology.

<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  Scaling Properties</span>

   The power and/or energy available to a device may vastly differ, from
   kilowatts to microwatts, from essentially unlimited to hundreds of
   microjoules.

   Instead of defining classes or clusters, we simply state, using the
   International System of Units (SI units), an approximate value for
   one or both of the quantities listed in Table 2:

   +------+--------------------------------------------------+---------+
   | Name | Definition                                       | SI Unit |
   +------+--------------------------------------------------+---------+
   | Ps   | Sustainable average power available for the      | W       |
   |      | device over the time it is functioning           | (Watt)  |
   |      |                                                  |         |
   | Et   | Total electrical energy available before the     | J       |
   |      | energy source is exhausted                       | (Joule) |
   +------+--------------------------------------------------+---------+

             Table 2: Quantities Relevant to Power and Energy





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   The value of Et may need to be interpreted in conjunction with an
   indication over which period of time the value is given; see
   <a href="#section-4.2">Section 4.2</a>.

   Some devices enter a "low-power" mode before the energy available in
   a period is exhausted or even have multiple such steps on the way to
   exhaustion.  For these devices, Ps would need to be given for each of
   the modes/steps.

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  Classes of Energy Limitation</span>

   As discussed above, some devices are limited in available energy as
   opposed to (or in addition to) being limited in available power.
   Where no relevant limitations exist with respect to energy, the
   device is classified as E9.  The energy limitation may be in total
   energy available in the usable lifetime of the device (e.g., a device
   that is discarded when its non-replaceable primary battery is
   exhausted), classified as E2.  Where the relevant limitation is for a
   specific period, the device is classified as E1, e.g., a solar-
   powered device with a limited amount of energy available for the
   night, a device that is manually connected to a charger and has a
   period of time between recharges, or a device with a periodic
   (primary) battery replacement interval.  Finally, there may be a
   limited amount of energy available for a specific event, e.g., for a
   button press in an energy-harvesting light switch; such devices are
   classified as E0.  Note that, in a sense, many E1 devices are also
   E2, as the rechargeable battery has a limited number of useful
   recharging cycles.

   Table 3 provides a summary of the classifications described above.





















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   +------+------------------------------+-----------------------------+
   | Name | Type of energy limitation    | Example Power Source        |
   +------+------------------------------+-----------------------------+
   | E0   | Event energy-limited         | Event-based harvesting      |
   |      |                              |                             |
   | E1   | Period energy-limited        | Battery that is             |
   |      |                              | periodically recharged or   |
   |      |                              | replaced                    |
   |      |                              |                             |
   | E2   | Lifetime energy-limited      | Non-replaceable primary     |
   |      |                              | battery                     |
   |      |                              |                             |
   | E9   | No direct quantitative       | Mains-powered               |
   |      | limitations to available     |                             |
   |      | energy                       |                             |
   +------+------------------------------+-----------------------------+

                   Table 3: Classes of Energy Limitation

<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>.  Strategies for Using Power for Communication</span>

   Especially when wireless transmission is used, the radio often
   consumes a big portion of the total energy consumed by the device.
   Design parameters, such as the available spectrum, the desired range,
   and the bitrate aimed for, influence the power consumed during
   transmission and reception; the duration of transmission and
   reception (including potential reception) influence the total energy
   consumption.

   Different strategies for power usage and network attachment may be
   used, based on the type of the energy source (e.g., battery or mains-
   powered) and the frequency with which a device needs to communicate.

   The general strategies for power usage can be described as follows:

   Always-on:  This strategy is most applicable if there is no reason
      for extreme measures for power saving.  The device can stay on in
      the usual manner all the time.  It may be useful to employ power-
      friendly hardware or limit the number of wireless transmissions,
      CPU speeds, and other aspects for general power-saving and cooling
      needs, but the device can be connected to the network all the
      time.

   Normally-off:  Under this strategy, the device sleeps such long
      periods at a time that once it wakes up, it makes sense for it to
      not pretend that it has been connected to the network during





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      sleep: the device reattaches to the network as it is woken up.
      The main optimization goal is to minimize the effort during the
      reattachment process and any resulting application communications.

      If the device sleeps for long periods of time and needs to
      communicate infrequently, the relative increase in energy
      expenditure during reattachment may be acceptable.

   Low-power:  This strategy is most applicable to devices that need to
      operate on a very small amount of power but still need to be able
      to communicate on a relatively frequent basis.  This implies that
      extremely low-power solutions need to be used for the hardware,
      chosen link-layer mechanisms, and so on.  Typically, given the
      small amount of time between transmissions, despite their sleep
      state, these devices retain some form of attachment to the
      network.  Techniques used for minimizing power usage for the
      network communications include minimizing any work from re-
      establishing communications after waking up and tuning the
      frequency of communications (including "duty cycling", where
      components are switched on and off in a regular cycle) and other
      parameters appropriately.

   Table 4 provides a summary of the strategies described above.

   +------+--------------+---------------------------------------------+
   | Name | Strategy     | Ability to communicate                      |
   +------+--------------+---------------------------------------------+
   | P0   | Normally-off | Reattach when required                      |
   |      |              |                                             |
   | P1   | Low-power    | Appears connected, perhaps with high        |
   |      |              | latency                                     |
   |      |              |                                             |
   | P9   | Always-on    | Always connected                            |
   +------+--------------+---------------------------------------------+

           Table 4: Strategies of Using Power for Communication

   Note that the discussion above is at the device level; similar
   considerations can apply at the communications-interface level.  This
   document does not define terminology for the latter.

   A term often used to describe power-saving approaches is "duty-
   cycling".  This describes all forms of periodically switching off
   some function, leaving it on only for a certain percentage of time
   (the "duty cycle").






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   [<a id="ref-RFC7102">RFC7102</a>] only distinguishes two levels, defining a Non-Sleepy Node
   as a node that always remains in a fully powered-on state (always
   awake) where it has the capability to perform communication (P9) and
   a Sleepy Node as a node that may sometimes go into a sleep mode (a
   low-power state to conserve power) and temporarily suspend protocol
   communication (P0); there is no explicit mention of P1.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Security Considerations</span>

   This document introduces common terminology that does not raise any
   new security issues.  Security considerations arising from the
   constraints discussed in this document need to be discussed in the
   context of specific protocols.  For instance, Section 11.6 of [<a href="#ref-COAP" title="&quot;Constrained Application Protocol (CoAP)&quot;">COAP</a>],
   "Constrained node considerations", discusses implications of specific
   constraints on the security mechanisms employed.  [<a href="#ref-ROLL-SEC-THREATS">ROLL-SEC-THREATS</a>]
   provides a security threat analysis for the RPL routing protocol.
   Implementation considerations for security protocols on constrained
   nodes are discussed in [<a href="#ref-IKEV2-MINIMAL">IKEV2-MINIMAL</a>] and [<a href="#ref-TLS-MINIMAL">TLS-MINIMAL</a>].  A wider
   view of security in constrained-node networks is provided in
   [<a href="#ref-IOT-SECURITY">IOT-SECURITY</a>].

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  Acknowledgements</span>

   Dominique Barthel and Peter van der Stok provided useful comments;
   Charles Palmer provided a full editorial review.

   Peter van der Stok insisted that we should include power terminology,
   hence <a href="#section-4">Section 4</a>.  The text for <a href="#section-4.3">Section 4.3</a> is mostly lifted from a
   previous version of [<a href="#ref-COAP-CELLULAR">COAP-CELLULAR</a>] and has been adapted for this
   document.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Informative References</span>

   [<a id="ref-COAP">COAP</a>]     Shelby, Z., Hartke, K., and C. Bormann, "Constrained
              Application Protocol (CoAP)", Work in Progress, June 2013.

   [<a id="ref-COAP-CELLULAR">COAP-CELLULAR</a>]
              Arkko, J., Eriksson, A., and A. Keranen, "Building Power-
              Efficient CoAP Devices for Cellular Networks", Work in
              Progress, February 2014.

   [<a id="ref-FALL">FALL</a>]     Fall, K., "A Delay-Tolerant Network Architecture for
              Challenged Internets", SIGCOMM 2003, 2003.








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   [<a id="ref-FIFTY-BILLION">FIFTY-BILLION</a>]
              Ericsson, "More Than 50 Billion Connected Devices",
              Ericsson White Paper 284 23-3149 Uen, February 2011,
              &lt;<a href="http://www.ericsson.com/res/docs/whitepapers/wp-50-billions.pdf">http://www.ericsson.com/res/docs/whitepapers/</a>
              <a href="http://www.ericsson.com/res/docs/whitepapers/wp-50-billions.pdf">wp-50-billions.pdf</a>&gt;.

   [<a id="ref-IKEV2-MINIMAL">IKEV2-MINIMAL</a>]
              Kivinen, T., <a style="text-decoration: none" href='https://www.google.com/search?sitesearch=datatracker.ietf.org%2Fdoc%2Fhtml%2F&amp;q=inurl:draft-+%22Minimal+IKEv2%22'>"Minimal IKEv2"</a>, Work in Progress, October
              2013.

   [<a id="ref-IOT-SECURITY">IOT-SECURITY</a>]
              Garcia-Morchon, O., Kumar, S., Keoh, S., Hummen, R., and
              R. Struik, "Security Considerations in the IP-based
              Internet of Things", Work in Progress, September 2013.

   [<a id="ref-ISQ-13">ISQ-13</a>]   International Electrotechnical Commission, "International
              Standard -- Quantities and units -- Part 13: Information
              science and technology", IEC 80000-13, March 2008.

   [<a id="ref-RFC0793">RFC0793</a>]  Postel, J., "Transmission Control Protocol", STD 7, <a href="./rfc793">RFC</a>
              <a href="./rfc793">793</a>, September 1981.

   [<a id="ref-RFC4838">RFC4838</a>]  Cerf, V., Burleigh, S., Hooke, A., Torgerson, L., Durst,
              R., Scott, K., Fall, K., and H. Weiss, "Delay-Tolerant
              Networking Architecture", <a href="./rfc4838">RFC 4838</a>, April 2007.

   [<a id="ref-RFC4919">RFC4919</a>]  Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
              over Low-Power Wireless Personal Area Networks (6LoWPANs):
              Overview, Assumptions, Problem Statement, and Goals", <a href="./rfc4919">RFC</a>
              <a href="./rfc4919">4919</a>, August 2007.

   [<a id="ref-RFC6550">RFC6550</a>]  Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R.,
              Levis, P., Pister, K., Struik, R., Vasseur, JP., and R.
              Alexander, "RPL: IPv6 Routing Protocol for Low-Power and
              Lossy Networks", <a href="./rfc6550">RFC 6550</a>, March 2012.

   [<a id="ref-RFC6551">RFC6551</a>]  Vasseur, JP., Kim, M., Pister, K., Dejean, N., and D.
              Barthel, "Routing Metrics Used for Path Calculation in
              Low-Power and Lossy Networks", <a href="./rfc6551">RFC 6551</a>, March 2012.

   [<a id="ref-RFC6606">RFC6606</a>]  Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem
              Statement and Requirements for IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Routing", <a href="./rfc6606">RFC</a>
              <a href="./rfc6606">6606</a>, May 2012.

   [<a id="ref-RFC7102">RFC7102</a>]  Vasseur, JP., "Terms Used in Routing for Low-Power and
              Lossy Networks", <a href="./rfc7102">RFC 7102</a>, January 2014.




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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-16" ></span>
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   [<a id="ref-ROLL-SEC-THREATS">ROLL-SEC-THREATS</a>]
              Tsao, T., Alexander, R., Dohler, M., Daza, V., Lozano, A.,
              and M. Richardson, "A Security Threat Analysis for Routing
              Protocol for Low-power and lossy networks (RPL)", Work in
              Progress, December 2013.

   [<a id="ref-RPL-DEPLOYMENT">RPL-DEPLOYMENT</a>]
              Vasseur, J., Ed., Hui, J., Ed., Dasgupta, S., and G. Yoon,
              "RPL deployment experience in large scale networks", Work
              in Progress, July 2012.

   [<a id="ref-TLS-MINIMAL">TLS-MINIMAL</a>]
              Kumar, S., Keoh, S., and H. Tschofenig, "A Hitchhiker's
              Guide to the (Datagram) Transport Layer Security Protocol
              for Smart Objects and Constrained Node Networks", Work in
              Progress, March 2014.

   [<a id="ref-V6-BTLE">V6-BTLE</a>]  Nieminen, J., Ed., Savolainen, T., Ed., Isomaki, M.,
              Patil, B., Shelby, Z., and C. Gomez, "Transmission of IPv6
              Packets over BLUETOOTH Low Energy", Work in Progress, May
              2014.

   [<a id="ref-V6-DECT-ULE">V6-DECT-ULE</a>]
              Mariager, P., Ed., Petersen, J., and Z. Shelby,
              "Transmission of IPv6 Packets over DECT Ultra Low Energy",
              Work in Progress, July 2013.

   [<a id="ref-V6-G9959">V6-G9959</a>] Brandt, A. and J. Buron, "Transmission of IPv6 packets
              over ITU-T G.9959 Networks", Work in Progress, May 2014.

   [<a id="ref-WEI">WEI</a>]      Shelby, Z. and C. Bormann, "6LoWPAN: the Wireless Embedded
              Internet", ISBN 9780470747995, 2009.



















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Authors' Addresses

   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   D-28359 Bremen
   Germany

   Phone: +49-421-218-63921
   EMail: cabo@tzi.org


   Mehmet Ersue
   Nokia Solutions and Networks
   St.-Martinstrasse 76
   81541 Munich
   Germany

   Phone: +49 172 8432301
   EMail: mehmet.ersue@nsn.com


   Ari Keranen
   Ericsson
   Hirsalantie 11
   02420 Jorvas
   Finland

   EMail: ari.keranen@ericsson.com






















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